/Engine/Plugins/Runtime/MeshModelingToolset/Source/ModelingComponents/Public/Scene/MeshSceneAdapter.h
Generate a new mesh that "caps" the mesh scene on the bottom. This can be used in cases where the geometry is open on the base, to fill in the hole, which allows things like mesh-solidification to work better. The base mesh is a polygon which can be optionally extruded. Currently the closing mesh is a convex hull, todo: implement a better option
void GenerateBaseClosingMesh ( double BaseHeight, double ExtrudeHeight )